Removes readme.md in favor of README.md b/c of colliding group

[khvatov]

While cloning the repo, git complains about 'colliding group is in the working tree'. There are two 'readme' files (one all upper case and one in lowercase) with identical contents. Removed readme.md in to stay consistent with other chapters' README.md.

[maprihoda]

Would it be possible to merge this PR? There really should NOT be two identical Readme files.

[ecerulm]

Just for completeness this is the warning that you get when trying to clone this repo in macOS:

git clone https://github.com/BasPH/data-pipelines-with-apache-airflow
Cloning into 'data-pipelines-with-apache-airflow'...
remote: Enumerating objects: 1492, done.
remote: Counting objects: 100% (2/2), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 1492 (delta 1), reused 0 (delta 0), pack-reused 1490
Receiving objects: 100% (1492/1492), 267.00 KiB | 2.67 MiB/s, done.
Resolving deltas: 100% (751/751), done.
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'chapter07/README.md'
  'chapter07/readme.md'

You can't have readme.md and README.md in a case-insentive filesystem like HFS+ of APFS found in macOS.

[krisgeus]

@khvatov This PR can be closed. We are working on a second edition and have fixed this in our separate fork which will be merged when the book is going to be published. Thx for noticing and taking the time to create this PR.